1. Field of the Invention
Embodiments of the present invention generally relate to an electrostatic deflection system used in electron beam systems.
2. Description of the Related Art
An electron beam is a group of electrons that have approximately the same kinetic energy and move in approximately the same direction. Electron beam technologies are used in many fields, such as cathode ray tubes (CRT), lithography, scanning electron microscopes, and welding. Electron beam systems, such as scanning electron microscopes, vector and raster beam lithography systems, usually have an electron beam column, in which at least one deflection system may be used to deflect electron beams, for example, to ensure the beams strike a target specimen at a precise location.
Electron beams are generally deflected by a magnetic or an electric field. An electrostatic deflection system is a system that uses an electric field to deflect the electron beams. Because an electric field is generally faster than an magnetic field in deflecting an electron beam, electrostatic deflection systems are usually used to implement fast deflection and to achieve high throughput in the electron beam systems. The miminal configuration for an electrostatic deflection system consists a capacitor that forms an electric field between two electrodes. The electron beam is deflected as it passes through this electric. Deflection signals are supplied to the capacitor in the form of an electric signal via a transmission line, for example, a high frequency 50 Ohm coaxial cable. The electric signal results in a voltage difference across the distance between two opposing electrodes of the capacitor. The deflection system is generally designed to minimize reflections of the electric signal due to impedance mismatch in the transmission line by selecting components to optimize impedance matching.
In the state-of-the-art deflection systems, impedance matching is done by making the capacitor a part of the transmission line, which means that the capacitor has the same impedance as the transmission line and the signal flows over the capacitor. FIG. 2 illustrates a schematic view of a state-of-the-art deflection system 200. Electrodes 202 and 212 are disposed opposing to one another and form a capacitor 210. The capacitor 210 is configured to deflect an electron beam 201 that passes through between the electrodes 202 and 212. A signal source 203 is adapted to supply a deflection signal to the electrode 202 via a drive coaxial cable 204. Particularly, the signal source 203 is connected to the electrode 202 via a core channel 204a of the drive coaxial cable 204 and an outer channel 204b of the drive coaxial cable 204 is grounded. The electrode 202 is further connected to the ground via a return coaxial cable 205 and a termination resistor 206. As used herein, termination resistor generally refers to any component having a desired impedance and need not necessarily be a resistor. The return coaxial cable 205 and the termination resistor 206 are connected in series. Similarly, the electrode 212 is connected to a signal source 213 via a drive coaxial cable 214 and connected to the ground via a return coaxial cable 215 and a termination resistor 216.
Usually, the drive coaxial cables 204, 214 and the return coaxial cables 205 and 215 are 50 Ohm coaxial cables, and the termination resistors 206 and 216 are equivalent to 50 Ohm resistors. The impedance between each electrode 202 or 212 and the ground is 50 Ohm respectively. Therefore, each electrode 202 or 212 acts as a part of transmission lines formed by the coaxial cables 204-205 and 214-215 respectively. The impedance of virtual coaxial cables 204-202-205 and 214-212-215 matches that of the termination resistors 206 and 216 respectively. The signal source 203 outputs a deflection signal that is inverted to a deflection signal from the signal source 213 such that the resultant voltage across the capacitor 210 to deflect the electron beam 101 is twice the amplitude of each of the signal sources 203 and 213.
During operation, the signal source 203 applies a voltage via drive coaxial cable 204. A current passes along the drive coaxial cable 204, the electrode 202 and the return coaxial cable 205, and then flows to the termination resistor 206. At point 207, the impedance of the virtual coaxial cables 204-202-205 matches the impedance of the termination resistor 206. Thus, reflection of wavefronts is minimized. Similarly, the signal source 213 sends a voltage (inverted of the voltage from signal source 203) down the drive coaxial cable 214. A current passes a long the drive coaxial cable 204, the electrode 212, the return coaxial cable 215 and the termination resistor 216. As discussed above, the impedance between each electrode 202/212 and the ground is 50 Ohm. Since the electrodes 202 and 212 are connected to a pair of inverted voltages, the voltage at the middle points between the electrodes 202 and 212 equals to the ground. Thus, the capacitor 210 may be considered as two capacitors (202-ground, and ground-212) in series each has an impedance of 50 Ohm. Therefore, the impedance of the capacitor 210 is 100 Ohm, or twice of that of the coaxial cable used. Because the impedance of the capacitor is given, among others, by the distance between the two electrodes of the capacitor, the impedance of the capacitor 210 can be adjusted by distance D1 between the electrodes 202 and 212.
FIG. 3 illustrates another state-of-the-art deflection system 200A. For ease of understanding, identical or similar features are identified by the same numerals in FIGS. 2 and 3. The deflection system 200A is similar to the deflection system 200 in FIG. 2. However, only one signal source 203 is used in the deflection system 200A. The electrode 212 is grounded. Therefore, the electrodes 202 and 212 form a capacitor 210A having an impedance matched to that of the coaxial cables 204 and 205. The distance between the electrodes 202 and 212 is D2, which is half of D1 in FIG. 2 if the same kind of coaxial cables are used in both systems.
However, the state-of-the-art electrostatic deflection systems discussed above have several disadvantages. First, because there is a current flows over the electrode into the termination resistor, eddy currents are induced in the neighboring metal areas when the current in the electrode changes with the deflection voltage. The induced eddy-current in turn creates a transient magnetic field which affects the beam deflection. The induced transient magnetic field drives the beam deflection angle causing the deflection system to lose precision, especially when the dwell time of the electron beam is larger than the eddy current transients. The induced magnetic field may also affect the electron beam with a long time constant so that the beam does not settle to a target position for a relatively long time, especially when the time constant of the eddy current is much larger than the dwell time of the beam at one point. Second, two connectors are required for each electrode increasing complexity of the system. Third, a rather large spacing is required between the capacitor electrodes in order to match the impedance of a high frequency cable, which has a standard impedance of 50 Ohm.
Since fast rising, high precision and simplicity are desirable in electron beam systems, a need exists in the art for a method and system for improving electron beam deflection.